Fix TimeSensor start_from_trigger causing dag_version churn

providers/standard/docs/sensors/datetime.rst

@@ -41,7 +41,13 @@ TimeSensor
 
 Use the :class:`~airflow.providers.standard.sensors.time_sensor.TimeSensor` to end sensing after time specified. ``TimeSensor`` can be run in deferrable mode, if a Triggerer is available.
 
-Time will be evaluated against ``data_interval_end`` if present for the Dag run, otherwise ``run_after`` will be used.
+The target moment is "today" (in the Dag's timezone) combined with ``target_time``, evaluated fresh
+each time the operator is instantiated -- which, for the default and for ``deferrable=True`` execution,
+happens close to the actual task run.
+
+``start_from_trigger`` is not supported by ``TimeSensor``: the target moment can only be correctly
+computed at task-execution time, not at Dag-parse time, so it cannot be handed to the triggerer ahead
+of time. Setting ``start_from_trigger=True`` raises a ``ValueError``.
 
 .. exampleinclude:: /../src/airflow/providers/standard/example_dags/example_sensors.py
     :language: python

providers/standard/src/airflow/providers/standard/sensors/time.py

@@ -64,6 +64,16 @@ def __init__(
     ) -> None:
         super().__init__(**kwargs)
 
+        if start_from_trigger:
+            raise ValueError(
+                "TimeSensor does not support start_from_trigger=True: the target "
+                "datetime is computed from the current wall-clock time at DAG-parse "
+                "time, not from the DagRun's data_interval_end/run_after, so baking "
+                "it into the serialized trigger arguments causes the DAG version to "
+                "change on every parse. Use deferrable=True without start_from_trigger, "
+                "or use DateTimeSensor with a templated target_time instead."
+            )
+
         # Create a "date-aware" timestamp that will be used as the "target_datetime". This is a requirement
         # of the DateTimeTrigger
 
@@ -77,14 +87,9 @@ def __init__(
         # Now that the dag's timezone has made the datetime timezone aware, we need to convert to UTC
         self.target_datetime = timezone.convert_to_utc(aware_time)
         self.deferrable = deferrable
-        self.start_from_trigger = start_from_trigger
+        self.start_from_trigger = False
         self.end_from_trigger = end_from_trigger
 
-        if self.start_from_trigger:
-            self.start_trigger_args.trigger_kwargs = dict(
-                moment=self.target_datetime, end_from_trigger=self.end_from_trigger
-            )
-
     def execute(self, context: Context) -> None:
         if self.deferrable:
             self.defer(

providers/standard/tests/unit/standard/sensors/test_time.py

@@ -135,3 +135,14 @@ def test_execute_complete_accepts_event(self):
             op.execute_complete(context={}, event={"status": "success"})
         except TypeError as e:
             pytest.fail(f"TypeError raised: {e}")
+
+    def test_start_from_trigger_raises_value_error(self):
+        """start_from_trigger bakes datetime.now() into trigger_kwargs at parse time,
+        causing the DAG version to change on every parse. See #69543."""
+        with DAG(
+            dag_id="test_start_from_trigger_raises",
+            schedule=None,
+            start_date=datetime(2020, 1, 1, 13, 0),
+        ):
+            with pytest.raises(ValueError, match="TimeSensor does not support start_from_trigger=True"):
+                TimeSensor(task_id="test", target_time=time(10, 0), start_from_trigger=True)